Metal salts

ABSTRACT

Alkali metal carboxylate salt brines, such as cesium formate brine, are used in oil and gas drilling procedures. Contamination with chloride ions can be controlled by treatment with a silver salt solution, and removing silver chloride formed. High density brines can be obtained, suitable for re-use.

The present invention concerns improvements in metal salts, moreespecially concerns the removal of certain ions from aqueous solutions.

It is commonplace in drilling for oil and gas, to use fluids such asbrines as well servicing fluids. In general, such fluids have desirablya high density. In particular, highly concentrated alkali metal salts ofcarboxylic acids have been developed for use in oil and gas welldrilling and completion operations. Such a fluid may be based uponcesium and potassium salts such as formates, which can provide brines ofspecific gravity values of 1.6 to 2.3, depending upon solutionconcentrations.

During use of such brines, they may be contaminated with sodium chloridefrom sea water or from water or salt entrained within the rock and mudduring application in the well bore. Chloride ions can cause severecorrosion of steel pipework and additionally may be incompatible withthe rock matrix, causing damage to or near the well bore area. Suchproblems may result in users of such brines refusing to re-usechloride-contaminated brines. A charge of cesium formate solution foruse as an oil field brine may cost in the order of $10M, so there isconsiderable need to improve the prospects for recycling such a brine.

Further, if a diluted brine is returned to high density by evaporationof water, any chloride concentration will increase by this process,thereby rendering the brine less suitable for re-use.

Accordingly, the present invention can be applied to removal of chloridefrom brine both caused by contamination in use or caused by aconcentration process such as evaporation, and throughout thisdescription and claims the term “brine” is to be understood as includingbrines which have been concentrated.

The present invention provides a method of treating an alkali metalcarboxylate salt brine contaminated with chloride ion, comprising mixingsuch contaminated brine with a solution of a silver salt, especiallysilver nitrate, causing silver chloride to be formed and separating thesilver chloride from the residual brine.

The invention may also be expressed as a method of use of an alkalimetal carboxylate salt brine, comprising the recovery of used brinecontaminated with chloride ion, treating the recovered brine with asolution of a silver salt, especially silver nitrate, causing silverchloride to be formed and separating the silver chloride from the brine,and re-using the brine. Desirably, the brine comprises cesium as themajority alkali metal by weight, and formate, acetate or other species,as the salt anion. The brine may comprise a significant proportion ofother alkali metals, and may therefore contain mixtures of alkali metalcarboxylates. Further, the brine may comprise one or more polymers orother components which are adjuvants and provide desirable properties tothe brine or avoid disadvantages.

The preferred silver salt is silver nitrate, and for ease ofdescription, all references hereinafter will be to silver nitrate.

Since the specific gravity of the brine is extremely important, it isdesirable to minimise any loss of specific gravity by undue dilutionduring the treatment. Accordingly, it is desirable to use silver nitratesolutions containing at least 200 g/l of AgNO₃, more preferably at least300 g/l AgNO₃ and most preferably at least about 800 g/l. At roomtemperature, saturation concentrations are about 1400 g/l AgNO₃. Thesilver nitrate solution may contain other components which do notsignificantly adversely interfere with the method of the invention, orthe performance of the brine. The silver nitrate solution isconveniently a product stream from a process involving the manufactureof high purity silver nitrate. This can provide economies arising fromheat and water savings and other processing costs.

The treatment of the invention is conveniently carried out at roomtemperature, but may be carried out at higher or lower temperatures. Itwill be borne in mind that solubility decreases with decreasingtemperature, and crystallisation may occur. Depending upon the chlorideconcentration, it may be necessary to cool the brine to about 0° C., tofacilitate the removal of sufficient nitrate salts formed as aby-product to create a stable solution that can be supplied and used inwinter conditions.

Conveniently, silver chloride may be removed from the brine byfiltration. This is particularly applicable because of the relativelyhigh value of the silver chloride. Other methods for separation may beused, however, such as hydrocyclones or centrifuges, and whereapplicable or desirable, polymeric materials may be added to enhanceseparation.

The treatment may be carried out in a two-step process, or, in apreferred embodiment, in a single step process. Desirably, at least a90% stoichiometric quantity of silver nitrate is used, relative tochloride ion, in the treatment. More desirably, the quantity of silvernitrate is from approximately 95% to approximately 112% stoichiometric,for brines containing 8.47 to 13.5 g/l chloride. If the brine has otherchloride contents or components, the stoichiometric adjustment maydiffer, which can be established by trial and error.

The recovered silver chloride may carry entrained by-products such assilver formate and cesium nitrate. Under preferred conditions, these areminimised and removed to permit the production of a re-usable brine. Bywashing and crystallisation, silver chloride crystals may be obtainedand the silver value recovered in conventional ways by conversion toother compounds or silver metal, using methods available to the personof ordinary skill in the art. In general, therefore, it is preferred toseparate the silver chloride by filtration, but conventional washing ofthe solids is not desirable if it causes undue dilution of the filtrate.Entrained cesium formate in the filtered precipitate may be washed outsubsequently, using water, and cesium nitrate may also be recovered ifdesired.

The skilled person may use the information herein to optimise theprocess, using conventional techniques.

The invention may be further understood with reference to the followingExamples.

EXAMPLE 1 Stoichiometric Additions of AgNO₃

200 ml samples of a used cesium formate brine, containing 1587 g/lcesium formate, 13.53 g/l chloride, 0.720 wt % sodium and 2.63 wt %potassium were used for all tests described. Desirably, the chloridelevel will be reduced below 1 g/l, more preferably 0.3-0.7 g/l.

In the first Example, stoichiometric amounts of AgNO₃ solution are addedwith stirring, to the brine, at room temperature and at differingconcentrations: AgNO₃ concn. (g/l) 150 200 600 918 1366 Volume added(ml) 87.6 65.7 21.6 14.5 9.5 Cl⁻ concn. (g/l) 0.50 0.64 0.72-0.99 1.01.78 (all concentrations in g/l normalised to 200 ml) Specific gravity1.848 1.913 2.084 2.117 2.129

In order to produce a product brine having Cl⁻ concentration ofapproximately 0.5 g/l, combined with a specific gravity of not less than2.0, a second stage treatment with AgNO₃ was undertaken, with thefollowing results: First stage product Cl⁻ (g/l normalised) 0.99 1.001.78 AgNO₃ stoichiometry (%) 50 50 80 AgNO₃ concn. (g/l normalised) 20090 200 Final Cl⁻ concn. (g/l normalised) 0.54 0.51 0.44 Overall AgNO₃stoichiometry (%) 103.2 103.4 110.0 Specific gravity 2.055 2.077 2.057After first and second stages, the deposits formed in the brine werefiltered off.

EXAMPLE 2 Single Stage Chloride Removal

The identical cesium formate brine as in Example 1 was used in furthertests, using differing stoichiometries: Test No. 1 2 3 AgNO₃stoichiometry (%) 107 110 116 Vol. AgNO₃ soln. added (ml) 10.2 10.4 11Cl⁻ concn. (g/l normalised) 0.58 0.35 <0.01 Specific gravity 2.111 2.1222.095Tests 1 and 2 proceeded satisfactorily at room temperature. It wasassessed in Test 3 that excess silver was being dissolved and apost-treatment of heating the product brine to 95° C. was incorporated,to remove the silver in solution.

A further post treatment of cooling to approximately 0° C. overnightfollowed by filtration, was found to remove a large proportion ofby-product cesium nitrate, leaving a stable clear solution at roomtemperature.

EXAMPLE 3 Singe Stage Chloride Removal from a K/Cs Formate Brine

200 ml samples of a potassium/cesium formate brine, containing 497 g/lformate and 8.47 g/l chloride, 0.675 wt % sodium, 12.56 wt % potassiumand 33.9 wt % cesium were used for all the tests described in thisExample 3. Desirably, the chloride level will be reduced such that itlies in the range 0.7 to 1.0 g/l.

An orthogonal array of sixteen tests in which different concentrationsand stoichiometric amounts of AgNO₃ are added at different temperatures,with different stirring rates, addition rates and residence times isshown below. In the tests in this Example, the reactants were stirredwith an IKA Werke RCT Basic stirrer, using either a “Slow” setting(setting 4) or a “Fast” setting (setting 7). Addition rate was either“Slow” or “Fast”, corresponding to a 15 minute addition time or a 30second addition time, respectively. Residence times are the times fromaddition of the last drop of AgNO₃ to the beginning of filtration, and“Short” means 5 minutes and “Long” means 30 minutes. Norm. AgNO₃Stoichio- Temp- Addi- Resi- Cl⁻ conc. metry erature Stir tion denceconc. Specific (g/l) (%) (° C.) Rate Rate Time (g/l) Gravity 600 95 25Fast Fast Short 0.68 1.859 600 100 25 Slow Fast Long 0.81 1.867 600 10535 Fast Slow Long 0.01 1.847 600 110 35 Slow Slow Short 0.38 1.848 80095 35 Slow Fast Long 1.61 1.867 800 100 35 Fast Fast Short 0.42 1.871800 105 25 Slow Slow Short 0.71 1.856 800 110 25 Fast Slow Long 0.011.851 1000 95 25 Slow Slow Long 1.86 1.856 1000 100 25 Fast Slow Short0.41 1.857 1000 105 35 Slow Fast Short 1.37 1.871 1000 110 35 Fast FastLong 0.01 1.868 1200 95 35 Fast Slow Short 0.87 1.884 1200 100 35 SlowSlow Long 1.84 1.886 1200 105 25 Fast Fast Long 0.14 1.871 1200 110 25Slow Fast Short 3.26 1.865

By following the orthogonal matrix, it can be demonstrated that thechloride level can be reduced from 8.47 g/l to between 3.26 and <0.1 g/lby the addition of AgNO₃ under various conditions.

Analysis of the chloride levels observed by following the orthogonalmatrix suggests that the stirring rate, concentration and stoichiometryof the AgNO₃ added, are the most significant factors that determine thepost-treatment chloride level in descending order of importance. Inparticular, a fast stirring rate is highly desirable for efficient AgClprecipitation as the formate reduction of silver side reaction isminimised. The temperature, addition rate and residence time appear tobe less significant factors.

To validate the conclusions drawn from the orthogonal array, a furtherthree confirmation tests were designed specifically to reduce thepost-treatment chloride level to 0.85, 0.65 and 0.45 g/l. The same brinewas treated at 35° C., with the AgNO₃ added slowly with fast stirringbefore a long residence time. Test No. 1 2 3 AgNO₃ stoichiometry (%) 9595 100 AgNO₃ concentration (g/l) 1200 800 1000 Target Cl⁻ level 0.850.65 0.45 Cl⁻ concentration 0.88 0.66 0.37 (normalised, g/l) SG 1.8901.874 1.884The most AgNO₃ efficient reduction in Cl⁻ level from 8.47 to <1 g/l andthe lowest amount of water added occurs in Test 1.

1. A method of treating an alkali metal carboxylate salt brine contaminated with chloride ion, comprising the steps of admixing the contaminated brine with a solution of a silver salt causing silver chloride to be formed in a reaction mixture and separating the silver chloride from the residual brine.
 2. A method according to claim 1, wherein the brine comprises cesium or cesium and potassium as the alkali metal(s) and formate, acetate or other species as the salt anion.
 3. A method according to claim 1, wherein the silver salt is silver nitrate and the concentration of the solution is at least 800 g/l AgNO₃.
 4. A method according to claim 1, wherein the silver salt is silver nitrate and the method further comprises the steps of cooling the reaction mixture then removing by-product alkali metal nitrate.
 5. A method according to claim 1, carried out such that the residual brine has a specific gravity of not less than 1.6.
 6. A method according to claim 1, wherein the silver salt is silver nitrate and silver nitrate is used in a quantity of from 95 to 112% of stoichiometric.
 7. A method of use of an alkali metal carboxylate salt brine, comprising the recovery of used or concentrated brine contaminated with chloride ion, treating the recovered brine with a solution of a silver salt causing silver chloride to be formed, separating the silver chloride from the brine, and re-using the brine.
 8. A method according to claim 1, wherein the silver salt is silver nitrate.
 9. A method according to claim 2, wherein the silver salt is silver nitrate and the concentration of the solution is at least 800 g/l AgNO₃.
 10. A method according to claim 4, wherein the cooling step comprises cooling the reaction mixture to about 0° C.
 11. A method according to claim 2, wherein the silver salt is silver nitrate and the method further comprises the steps of cooling the reaction mixture then removing by-product alkali metal nitrate.
 12. A method according to claim 11, wherein the cooling step comprises cooling the reaction mixture to about 0° C.
 13. A method according to claim 2, carried out such that the residual brine has a specific gravity of not less than 1.6.
 14. A method according to claim 2, wherein the silver salt is silver nitrate and silver nitrate is used in a quantity of from 95 to 112% of stoichiometric.
 15. A method according to claim 7, wherein the silver salt is silver nitrate. 